Dispersions of water insoluble organic materials, such as dye particles, find uses in many areas of technology, one of the more notable of which is color photography. These dispersions are commonly made by milling the dye material with a carrier fluid and a dispersant.
Dye particles are generally employed in a crystalline habit, which include needles and platelets. It has been shown by the present invention that if the dye particles can be made spherical, handling advantages are obtained. For example, lower viscosities result from dispersions of spherical particles and such dispersions have greater stability than dispersions of needles, platelets and other non-compact habits. Non-dye materials, such as, for example, methyl phenyl hydroquinone may also be recovered in a spherical habit by the present invention.
It is apparently a property of crystalline materials to initially precipitate as spheres. In their study of inorganic sols, Berestneva et al. [Kolloid. Zh. 12, 338 (1950); 13 323 (1951); 14, 395 (1952); USP. KHIM. 24, 249 (1955)] found that only upon standing in a solvent medium would initially precipitated spherical particles recrystallize to their predominately stable crystal habit. It is theorized that when a particle is initially precipitated from solution it does not have time to form as well ordered crystal, but instead is composed of a random array of crystallites. Because of surface forces these crystallites tend to associate in a spherical habit.
It has been discovered and claimed in the copending U.S. patent application of Edgar Gutoff, Ser. No. 793,062, filed on the same day hereof and assigned to Polaroid Corporation, that stable dispersions of spherical particles of organic materials such as dyes can be formed by rapidly precipitating the material from an organic solution and quickly removing solvent thereby resulting in a stable dispersion of spherical material particles. All this is preferably done in the presence of a suitable dispersing agent to prevent agglomeration of the particles. This is also preferably accomplished in a continuous system under constant conditions whereby residence times can be adjusted to provide accurate control of the size of the resultant spherical particles. The process of the Gutoff invention is applicable to any organic material which is substantially water insoluble and soluble in an organic solvent miscible with and more volatile than water.
The present invention is an improvement over the above described Gutoff process and requires that the organic solvent stream of the Gutoff process contain at least ten percent by weight water but less than the amount of water which would cause the dissolved organic material to precipitate from solution. By this method large particles are minimized and controlled dispersions with particles less than 1 micron in size are prepared.